An optical signal transmitter is typically constructed from a laser followed by a modulator that modulates the coherent radiation emitted by the laser. The laser may be a laser diode (LD), and the modulator may be a semiconductor electro-absorption modulator (EAM). Typically, the LD-EAM combination is required to output a high radiation power and is also required to have a high extinction ratio (ER) and relatively flat frequency response up to the bit rate frequency.
U.S. Pat. No 6,381,066 to Korn, et al., whose disclosure is incorporated herein by reference, describes an integrated semiconductor optical amplifier (SOA) system. The system consists of a single SOA which is mounted on an optical bench, with fiber optics coupling optical radiation into and out of the SOA via isolators. The system is constructed, with a diode that monitors radiation in the system, as a hermetic package.
U.S. Pat. No. 6,400,864 to Lee, whose disclosure is incorporated herein by reference, describes a broadband SOA module. An broadband optical signal is demultiplexed by a demultiplexer into separate wavelengths, and each wavelength is amplified by a respective SOA. The amplified wavelengths are then combined in a multiplexer. The demultiplexer, the SOAs, and the multiplexer are integrated on a single semiconductor substrate.
To achieve a flat frequency response, typical optical transmitters utilize a matched circuit that may include capacitors and that is tuned to the EAM. In order to flatten the frequency response curve the circuit reduces the response of the EAM by a relatively large amount at lower frequencies, thus effectively sacrificing some of the available extinction ratio of the EAM. Other parameters that may adversely affect the output power and signal quality of the LD-EAM combination include the insertion loss (IL) of the modulator, and chirp of the EAM. In LD-EAM transmitters known in the art the output power is limited by the power handling ability of the EAM.
An optical radiation transmitter which outputs a high radiation launch power, with minimal sacrifice in ER, and which has reduced chirp, would thus be advantageous.